Electronic devices with barium barrier film and process for making same

ABSTRACT

A semiconductor device having a barrier film comprising an extremely thin film formed of one or more monolayers each comprised of a two-dimensional array of metal atoms. In one exemplary aspect, the barrier film is used for preventing the diffusion of atoms of another material, such as a copper conductor, into a substrate, such as a semiconducting material or an insulating material. In one mode of making the semiconductor device, the barrier film is formed by depositing a precursor, such as a metal halide (e.g., BaF 2 ), onto the substrate material, and then annealing the resulting film on the substrate material to remove all of the constituents of the temporary heteroepitaxial film except for a monolayer of metal atoms left behind as attached to the surface of the substrate. A conductor, such as copper, deposited onto the barrier film is effectively prevented from diffusing into the substrate material even when the barrier film is only one or several monolayers in thickness. The extremely thin barrier film makes possible a significant increase in the component density and a corresponding reduction in the number of layers in large scale integrated circuits, as well as improved performance.

STATEMENT OF GOVERNMENT INTEREST

[0001] The invention described herein may be manufactured and used by orfor the Government of the United States of America for Governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to the fabrication of electronicdevices, and particularly to a novel barrier film for electronic andelectro-optic materials.

[0003] Integrated circuits (ICs) are composed of many millions(sometimes billions) of components such as transistors, resistors, andcapacitors. These individual components are laid out in a twodimensional array on a substrate such as silicon or gallium arsenide.The two dimensional arrays are often stacked one on top of another toform a three dimensional IC. As in any circuit, these components, andthe several layers, must be connected to one another electrically.Interconnection on the two dimensional surfaces is accomplished bydepositing strips of metal that act as connecting “wires.” Likewise, thelayers are interconnected by metal plugs deposited in via holes madebetween layers. These steps in the manufacturing process are commonlyreferred to as “metallization.”

[0004] Generally, silicon is the substrate material of choice, aluminumis the metal of choice for two dimensional IC metallization, andtungsten is the metal of choice for filling via holes for multiple layerinterconnection. Silicon is preferred because it is cheap and abundant.Aluminum and tungsten are chosen because they have adequate electricalconductivity and they can be made not to diffuse into the substrateduring the many annealing operations inherent in the IC manufacturingprocess.

[0005] Because the electrical conductivity of aluminum and tungsten islimited, the “wires” and plugs must be made thick enough to ensureminimal resistance to electric current between components and betweenlayers. The large size of these conductors has recently become an issuefor IC designers and fabricators interested in placing a greater densityof circuit elements on an IC. In order to achieve greater performancefrom ICs, the lateral dimensions of the circuit elements must bereduced. This reduction in IC element size has two detrimental effectson the resulting IC. First, it increases the resistance of the metalinterconnects. Second, it increases the aspect ratio of the via holes,making them more difficult to fill with the metallic material.Incomplete filling of the via holes exacerbates the problem of highresistance. Today, there is often not enough space in the lateraldirection on an IC chip to accommodate large aluminum conductors.Additionally, the size of the via holes, when filled with tungsten,limits the number of levels in the IC to no more than five.

[0006] Copper, which is a much better conductor of electricity thanaluminum, is available as an alternative metallization material. Becauseof copper's greater electrical conductivity, copper imposes lessresistance to the flow of electrons than aluminum or tungsten conductorshaving equivalent dimensions. The increasing density of components ontoday's ICs requires the smaller sized conductors that are onlyachievable by the use of highly conductive metallization materials.

[0007] Unfortunately, copper has one notable problem. It has a tendencyto diffuse into silicon at elevated temperatures. This has precludedcopper as a metallization candidate because ICs must be annealed severaltimes during the manufacturing process. In order for coppermetallization to be feasible, a technique must be developed that willprevent the diffusion of copper into silicon. Among the possiblesolutions currently under development within the semiconductor industrythe most prevalent is the use of nitrides of the transition metalstitanium and tungsten. The thickness of the metal-nitride layer requiredto stop copper diffusion into silicon effectively is in the range oftens to hundreds of nanometers, or hundreds to thousands of Angstroms(Å).

[0008] The problem of diffusion exists not only in the case of coppermetallization on silicon, but also in the case of copper metallizationon other single- and polycrystalline semiconductor substrate materialssuch as gallium arsenide, silicon carbide, germanium, and so forth.Copper diffusion into insulating materials such as SiO₂ can also resultin short circuits, especially in dense arrays of IC components.Diffusion is also a problem with other high conductivity metallizationmaterials such as gold, silver, and platinum.

[0009] An object of this invention is to provide a barrier film which isextremely thin, yet permits metallization using copper and other highconductivity metallic conductors which would otherwise have a tendencyto diffuse into a substrate formed of a semiconducting or insulatingmaterial.

[0010] It is also an object of the invention to improve electronic andelectro-optic devices by making it possible to achieve one or more ofthe following desirable characteristics: increased component density inlarge scale integration, reduced heat dissipation, increased speed ofoperation, and a decreased number of layers.

[0011] Still another object is to provide a procedure for forming anextremely thin diffusion barrier, which produces consistent resultsrapidly and reliably, and which is not highly dependent upon theaccurate maintenance of operating conditions such as time andtemperature.

[0012] Still another object is to provide a process for forming anextremely thin diffusion barrier which eliminates voids and mechanicalstresses that can have detrimental effects on the substrate, thediffusion barrier, or the metallization layer.

SUMMARY OF THE INVENTION

[0013] In accordance with this invention, a semiconductor device isfabricated by forming, on a surface of a substrate material, a barrierfilm having a monolayer of metal atoms immediately adjacent the surfaceof the substrate material. In one aspect, a metallic conductor, whichhas a tendency to diffuse into the substrate material, is then depositedonto the barrier film. Metallic conductors which have a tendency todiffuse into substrates of semiconductor or insulating materialsinclude, for example, pure copper, copper alloys (e.g., Cu—Al,Cu—Si—Al), copper doped with a dopant (e.g., aluminum) that impedeselectromigration, gold, silver, or platinum. For purposes of thisinvention, a “monolayer” is understood to refer to a two-dimensionalarray of atoms having the thickness of one atomic layer; although themonolayer may have minuscule defects such as minute portions with athickness that exceeds one atomic layer and/or minute portions that arevoids, the average thickness nonetheless essentially is an atomic layerproviding essentially complete coverage of the directly underlyingsubstrate surface regions. The monolayer, which is extremely thin bydefinition, serves as a barrier film, inhibiting diffusion of themetallic conductor into the substrate material. For purposes of thisapplication, the material upon which the monolayer of atoms is formed isoften generally referred to herein as a “substrate” for such formation,and it will be appreciated that the term “substrate” as used herein canencompass a bulk wafer or, alternatively, a layer that is grown,deposited, formed or bonded upon another body. The present invention isespecially concerned with substrates that are semiconductor orinsulating materials.

[0014] In one preferred method of this invention, a monolayer isproduced by depositing a metal halide upon a surface of a semiconductingor insulating substrate material where it first reacts with thesubstrate material and dissociates, releasing gaseous by-products formedof substrate atoms and halogen atoms of the precursor compound. Thisreaction is self-limiting resulting in formation of a monolayer of metalatoms on the substrate that thereafter enables a homoepitaxial filmformed of the metal halide molecules to form thereon as the depositionprocess proceeds. This deposition operation can be carried out byvarious methods, but is preferably carried out by molecular beamepitaxy, or alternatively by r.f. sputtering. At this juncture, atemporary heteroepitaxial film has been formed on the substrate wherethe diffusion barrier is ultimately desired. Then, in a second stage ofthe procedure, the temporary heteroepitaxial film is subjected to aselective removal procedure, whereby the homoepitaxial portion of thedeposited film having the halogen constituents is selectively eliminatedwhile the monolayer of metal atoms remains behind attached to thesurface of the substrate material. The removal procedure preferably isan annealing operation. Alternatively, chemical etching which isselective to remove the homoepitaxial portion of the deposited filmwhile leaving the monolayer of metal atoms also can be used. In anyevent, the metal atom monolayer strongly adheres to the substratematerial, and is not adversely affected by extended annealing times,high annealing temperatures, or chemical etching conditions.

[0015] The precursor compound preferably comprises a metal halide, e.g.,a barium, strontium, cesium or rubidium- halide salt. The thickness ofthe monolayer basically corresponds to the diameter of the metal atomconstituent(s) of the monolayer. Metal atoms of barium, strontium,cesium, rubidium, and so forth have a thickness (i.e., the diameter ofthe largest electron orbital) of less than 5 Å, so it can be appreciatedthat an extremely thin diffusion barrier layer is achieved by thisinvention. The semiconducting substrate materials that can be processedaccording to this invention include mono- or polycrystalline, doped orundoped, semiconductors, such as silicon, germanium, indium phosphide,gallium arsenide, silicon carbide, gallium nitride, aluminum nitride,indium antinomide, lead telluride, cadmium telluride, mercury-cadmiumtelluride, lead selenide, lead sulfide, tertiary combinations of thesematerials, and so forth. The insulating substrate materials that can beprocessed according to this invention include doped or undoped siliconoxides (e.g., silicon dioxide), silicon nitride, phosphosilicate glass(PSG), borophosphosilicate glass (BPSG), barium fluoride, strontiumfluoride, calcium fluoride, and so forth.

[0016] In a further embodiment of this invention, a multiplicity ofmonolayers are formed contiguous with each other upon the substratesurface. This embodiment can become advantageous such as where asubstrate is involved having a relatively greater surface roughness andit is necessary to account for any discontinuities in the surfaceprofile by sufficiently building-up the diffusion layer to blanket thesurface topography presented and provide complete coverage. To build-upmultiple monolayers, one stacked upon the other, MBE deposition can beused to sequentially deposit additional monolayers of metal atoms usingan elemental source of the metal. In this way, a diffusion barrier filmthickness can be assembled up to any desired thickness, but preferablyis maintained at or below not more than 100 Å, more preferably not morethan 20 Å to meet the primary objective of providing an extremely thinyet effective diffusion barrier.

[0017] As can be appreciated, a semiconductor device is obtained by thisinvention in which a monolayer or several monolayers of metal atomsseparates a metallic conductor from other materials in the device, suchas semiconductor or insulating materials, in which the extremely thindiffusion barrier film serves as an effective barrier preventing atomsof the metallic conductor from diffusing into such other materials andeither impairing the device or rendering it totally inoperative.

[0018] Various other objects, details and advantages of the inventionwill be apparent from the following detailed description when read inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic cross-section depicting diffusion of copperinto a silicon substrate, where no diffusion barrier is present;

[0020]FIG. 2 is a graph illustrating the projected requirement indiffusion barrier thickness by the Semiconductor Industry Association;

[0021]FIG. 3 is a schematic cross-section depicting the effect of adiffusion barrier in accordance with the invention;

[0022]FIG. 4 is a schematic diagram illustrating the process ofdeposition of a diffusion barrier precursor compound, and ametallization layer, onto a substrate by molecular beam epitaxy;

[0023] FIGS. 5A-E is a schematical illustration showing the interfacialstructure of the diffusion barrier on an atomic level as it is beingformed on a semiconductor substrate after various process stepsaccording to an inventive process;

[0024]FIG. 6 is a schematic diagram illustrating the process ofdeposition of a diffusion barrier precursor compound onto a substrate byr.f. sputtering; and

[0025]FIG. 7A is a schematical illustration showing the interfacialstructure of the barrier film on an atomic level where the barrier filmis comprised of a plurality of contiguous monolayers, while FIG. 7Bshows another embodiment of the invention where the barrier film is acomposite monolayer formed of different types of metal atoms, and FIG.7C shows yet another embodiment where the barrier film is comprised of aplurality of contiguous monolayers in which different monolayers thereofare formed of different types of metal atoms.

[0026]FIG. 8 is a schematic cross-sectional view showing a diffusionbarrier in accordance with the invention preventing diffusion of acopper plug into silicon substrate and into a silicon dioxide insulatinglayer overlying the substrate.

DETAILED DESCRIPTION

[0027]FIG. 1 illustrates a typical attempt at copper metallization of asilicon semiconductor substrate 10. The substrate, which is made up ofsilicon atoms 12, has two laterally delineated copper interconnectstrips 14 deposited on its surface. In the annealing process, copperatoms 16 tend to diffuse into the substrate, impairing itssemiconducting properties, and usually rendering it totally inoperative,by effectively creating an electrical short circuit. Similar diffusionoccurs at an interface between a copper conductor and a SiO₂ insulatinglayer, for example in the case in which an attempt is made to deposit aconducting copper plug in a via hole in the SiO₂ insulating layer.Diffusion of copper atoms into the SiO₂ insulating layer impairs itseffectiveness as an insulator and may have a serious adverse effect onthe properties of the device.

[0028] As mentioned above, various attempts have been made to achieve adiffusion barrier to permit the use of copper conductors insemiconductor devices. The most attention so far has been given to theuse of nitrides of tungsten and titanium. Various other diffusionbarrier materials, for example tantalum nitride, have also been tried.As shown in FIG. 2, which is based on published Semiconductor IndustryAssociation data, presently achievable diffusion barrier thicknesses areonly in the 200-250 Å range for tantalum nitride, although thicknessessmaller than that are expected to be pursued by the industry in theupcoming years.

[0029] This invention provides an effective diffusion barrier having athickness well below 100 Å, and below 5 Å in one exemplary embodiment,which is far below the minimum thickness projected by the industry datadepicted in FIG. 2. The extremely thin diffusion barrier layersachievable by this invention potentially could be useable long afteralternative technologies become obsolete.

[0030] Referring now to FIG. 3, a portion of an integrated circuit isschematically illustrated on an atomic level that comprises a siliconsubstrate 18 made up of silicon atoms 20, and having laterallydelineated copper interconnect strips 22. At the upper surface of thesilicon substrate 18, a monolayer of barium (Ba) atoms 24 is interposedbetween the conductor strips 22 and the surface of the substrate 18 andeffectively prevents diffusion of the copper atoms into the silicon. Thelayer of Ba atoms need only have a thickness of one atomic layer, i.e.,a monolayer of approximately 5×10⁻¹⁰ meters (5 Å) in thickness, in orderto provide the desired barrier to diffusion of the conductor into theadjoining substrate. The barium layer depicted in FIG. 3 illustrates thesituation in which a single monolayer of barium is provided. Theextremely small thickness of the diffusion barrier contributes to thereduction in both thickness and the lateral dimensions of the integratedcircuit layer, and the ability to use copper interconnects and otherconductor materials otherwise predisposed to creating diffusion problems(e.g., Au, Ag, Pt) instead of aluminum interconnects. As such, thepresent invention represents a remarkable breakthrough in the field.

[0031] As will become apparent from the following description, in onemode of this invention, a diffusion barrier comprised of metal atoms andhaving a thickness of not more than approximately 5 Å is achievable bydepositing a metal halide precursor compound on a semiconductor orinsulating substrate so as to form a temporary heteroepitaxial filmthereon. Then, the resulting temporary heteroepitaxial film created bythe metal halide and the substrate surface is subjected to a post-growthanneal or chemical etching in which all of the temporary heteroepitaxialfilm is eliminated by removal from the substrate except for an atomiclayer of the metal component, i.e., a monolayer. This residual monolayerof metal atoms disposed in contact with the substrate surface provides adiffusion barrier to conductor materials.

[0032] One suitable approach for depositing the metal halide used as aprecursor compound for forming the diffusion barrier layer is molecularbeam epitaxy (MBE), such as depicted in FIG. 4. A substrate 26, e.g., asilicon wafer, is supported on a rotating holder 28 within aconventional MBE deposition chamber 30. The deposition chamber isillustrated in simplified form. Not shown are provisions for raising thetemperature of the substrate to annealing temperatures and forevacuating the chamber. Also not shown is a conventional Reflective HighEnergy Electron Diffraction (RHEED) diagnostic system directed towardthe substrate 26.

[0033] A diffusion barrier precursor compound effusion cell, for examplea barium fluoride, strontium fluoride or the like effusion cell, isprovided at 32, and has a shutter 33. A shutter 35 is also provided forthe silicon wafer 26. An electron beam source for the metallizationlayer, e.g., copper, is shown at 34.

[0034] In the operation of the MBE deposition apparatus of FIG. 4, thesubstrate 26 is placed inside the chamber 30 and positioned by rotatableholder 28 and the chamber 30 is evacuated, using ion pumps and liquidnitrogen trapping to achieve a high vacuum. The substrate 26 is vacuumannealed to remove any passivation layer by deoxidation, for examplesilicon dioxide in the case of a silicon wafer.

[0035] The temperature of the substrate 26 is then reduced to a suitabledeposition temperature, and the effusion cell 32 is heated while thesubstrate 26 is mechanically rotated. The electron beam of a RHEEDdiagnostic system is focused onto the substrate 26 and the RHEED patternis monitored. When the RHEED pattern corresponding to the single crystalsubstrate surface appears (indicating complete removal of thepassivation layer) on the RHEED screen, the shutters 35 and 33 in frontof the substrate holder 28 and the effusion cell 32, respectively, areopened to allow precursor molecules to impinge on the substrate surface29. Deposition of the precursor 27 onto the silicon surface 29 begins,and is allowed to continue until the single crystal silicon RHEEDpattern disappears and is replaced by a pattern corresponding to asingle crystal layer of the precursor compound. Deposition is halted byclosing the substrate and effusion source shutters 35 and 33,respectively. By this juncture, a temporary heteroepitaxial film derivedfrom the precursor molecules is situated on the substrate surface 29,although the nature of the interface is more complicated as will becomeapparent from later descriptions herein.

[0036] During the deposition of the precursor 27 on the substrate 26,the substrate 26 should be at a temperature in the range fromapproximately 500° C. to 800° C., and ideally at approximately 750° C.,though the temperature will vary depending on the particular substrateand the processing tool. The pressure within the deposition chamber 30should be 10⁻⁸ mbar or less, more preferably 10⁻⁹ mbar or less, andstill more preferably 10⁻¹⁰ mbar or less, in the case of depositing ametal halide on a silicon substrate. The time required to achieveadequate deposition of the precursor sufficient to form the temporaryheteroepitaxial film on the substrate is typically one or two minutes,but is not limited thereto.

[0037] Following the deposition on the substrate of the temporaryheteroepitaxial film derived from the precursor compound, thetemperature of the substrate is raised to cause precursor molecules todetach from the temporary heteroepitaxial film on the substrate. In thecase of BaF₂, barium atoms adjacent to the substrate remain tightlyadhered thereto as a two-dimensional monolayer, while fluorine atoms (asbonded to other barium atoms) in the temporary heteroepitaxial film areeffectively re-evaporated in the form of barium fluoride and eliminatedfrom the temporary heteroepitaxial film. This will cause the RHEEDpattern to change in appearance. Specifically, the “reappearance” of aRHEED pattern similar to that for the single crystal substrate confirmsthat the precursor molecules have been evaporated. The substratetemperature at which the detachment of the precursor molecules with thehalogen atoms from the temporary heteroepitaxial film takes place duringthe post-growth anneal step is not necessarily limited, but should be inthe vicinity of 750° C. to 1000° C., preferably 800° C. The monolayer ofmetal atoms which remains on the substrate serves as the diffusionbarrier between the substrate and any metallization layer subsequentlydeposited upon it. The metallization layer can be deposited by any ofvarious standard microelectronic metallization methods, and, in thisembodiment, it can be conveniently deposited while the substrate isstill in the MBE chamber by operation of the electron beam source.

[0038] The crystallographic and chemical characterization of theaforesaid temporary heteroepitaxial film, and the effect of treatmentsthereof according to this invention to form a diffusion barrier film onthe substrate, are now discussed in greater detail. Based on X-rayphotoelectron spectroscopy (XPS) and heavy ion backscatteringspectroscopy (HIBS) analyses of the precursor compound/substrate surfaceinterfacial chemistry, the formation of an ultra-thin metal monoatomiclayer (monolayer) on the substrate is considered to proceed by amulti-stage process, which is schematically illustrated in FIGS. 5A-E.XPS and HIBS analysis measurements referred to herein can be performedusing generally available equipment and analyses protocol understood andimplementable by one skilled in the art.

[0039] As schematically illustrated in FIG. 5A, BaF₂ molecules 50 aredirected and impinged onto the surface 51 of a silicon substrate 52,such as by MBE deposition. For FIGS. 5A-E, BaF₂ is used to illustratethe metal halide, and silicon is used to illustrate the (semiconductor)substrate, although other materials can be used as indicated elsewhereherein.

[0040] Ideally, the silicon surface 51 to be used as the depositionsubstrate has a highly planar, smooth surface to minimize the coatingthickness needed to provide complete coverage thereof. Deoxidationannealing, chemical-mechanical-planarization (CMP) polishing or ionmilling can be used in a pretreatment of the silicon surface prior todeposition of the diffusion barrier to enhance the planarity andsmoothness of silicon surface, if necessary. On the other hand, as willbe described below, the inventive process itself provides some measureof in situ planarization of the silicon surface during MBE deposition.

[0041] In any event, in the first step, the BaF₂ molecules react withsilicon atoms 51 a, 51 b, 51 c, and so forth, at the surface 51 of thesilicon substrate 52. The Ba—F and silicon-silicon bonds at the surfaceof the silicon substrate are broken. As schematically shown in FIG. 5B,the free silicon and fluorine atoms at the vicinity of the interfacewhere the barium fluoride molecules are contacting the silicon surface51 then combine to form volatile silicon-fluoride compounds (SiF_(y)) 53which escapes from the silicon substrate surface 51, and it is extractedfrom the MBE chamber via vacuum. Although FIG. 5B depicts compound 53 astwo halide atoms (white circles) bonded to a common metal atom (darkenedcircle), it will be understood that this illustrative only because othergaseous metal-halides may be generated, such as tetrahalides of siliconwhere the substrate 52 is silicon. By comparison, if the substrate 52instead is GaAs, the escaping gas 53 would be GaF. This etching-likeeffect upon the surface silicon atoms serves to effectively smoothen thesilicon surface.

[0042] In any event, as illustrated in FIG. 5B, the barium atoms leftbehind bond with dangling bonds of the surface silicon atoms, forming amonoatomic layer 54 of metal atoms, i.e., a metal monolayer of bariumatoms. This deposition step proceeds for a sufficient duration of timeto form a continuous layer of barium atoms across the surface of thesilicon substrate without leaving any bare spots.

[0043] As illustrated in FIG. 5C, once complete coverage of the siliconsubstrate 52 with barium atoms 54 is achieved, barium fluoride 50deposition via MBE is continued from a molecular beam. As illustrated inFIG. 5D, this subsequently introduced barium fluoride adheres to thebarium monolayer 54 and grows epitaxially thereon to form a temporaryhomoepitaxial film portion 55. The amount of subsequent deposition ofepitaxial barium fluoride on the barium monolayer is allowed to beenough to provide a safety measure which ensures complete substratecoverage with a monolayer of barium atoms. In this way a heteroepitaxialfilm 56 is formed on the substrate surface 51 comprising a monolayer 54of metal (e.g., Ba) atoms as an interaction regime attached directly tothe substrate surface 51 and a homoepitaxial regime 55 comprised oforiented molecular metal halide (e.g., barium fluoride) formed, in turn,on the monolayer 54. The homoepitaxial regime 55 of BaF₂ of thetemporary heteroepitaxial film 56 is (100)-oriented on silicon (100),and (111)-oriented when the substrate is silicon (111), GaAs (100), orGaAs (111).

[0044] XPS measurements have confirmed that barium atoms have the twoabove-mentioned different chemical states, i.e., the interaction (metalmonolayer) and the homoepitaxial regimes, in the temporary film presentat this stage of processing. The relative abundance of these two stateshas also been determined by XPS. The number of barium atoms in eachstate is determinable by normalizing integrated XPS peak intensities toHIBS measurements of the total number of barium atoms on the surface.The results of these analyses confirm that BaF₂ first reacts with thesilicon surface during initial MBE deposition at the silicon surface anddissociates, releasing a gaseous silicon-fluorine compound. Thisreaction is self-limiting, resulting in a barium monolayer that enablessubsequent BaF₂ molecules to form an epitaxial (111)-oriented film onthe silicon surface. Then, a post-growth anneal affects evaporation ofthe barium fluoride deposited on the monolayer.

[0045] That is, as illustrated in FIG. 5E, in a second stage of thisinventive procedure, which is conducted after the MBE deposition of themetal halides on a substrate to form the temporary heteroepitaxial film56 shown in FIG. 5D, a vacuum anneal is performed to cause evaporationof barium fluoride 57 from the temporary heteroepitaxial film such thatthe barium fluoride content found in the homoepitaxial portion thereof(feature 55 in FIG. 5D) is completely removed back to the monolayer 54of barium atoms attached to the silicon surface 51. Alternatively, thehomoepitaxial portion 55 of the temporary heteroepitaxial film can beremoved by etching (e.g., chemical etching) which is selective betweenthe homoepitaxial portion 55 and the monolayer portion 54 such that theformer can be removed while leaving the latter intact.

[0046] In any event, prior to performing the post-growth anneal (oretching) to remove the homoepitaxial portion 55 of the temporaryheteroepitaxial film, there is no practical limit on how thick theoverdeposit of barium fluoride can be that is formed over the bariummonolayer. However, it will be appreciated that the thicker thedeposited barium fluoride layer(s) of the homoepitaxial portion of thetemporary film is made to be, the longer the post-growth anneal timethat will be necessary to decompose the deposited thickness of bariumfluoride molecules back to the monolayer of barium atoms left attachedto the substrate surface.

[0047] The MBE deposition of the temporary heteroepitaxial film and thepost-growth anneal can be performed in the same processing chamberwithout breaking the vacuum between the two procedures. Alternatively,the MBE deposition can be performed in a first processing tool, afterwhich the vacuum is broken, and the workpiece is then transferred toanother processing tool for separately performing the post-growth annealat which time the substrate is heated up again with a vacuum beingcreated in the second processing tool. In the latter case, thehomoepitaxial portion of the temporary heteroepitaxial film serves as aprotective coating over the monolayer portion of the heteroepitaxialfilm during such transit between separate processing tools.

[0048] In that the atomic diameter of barium is 4.48 Å, and that ofstrontium is 4.29 Å, it can be appreciated how the formation of amonolayer of these metal atoms, for example, on a substrate by thetechniques presented herein permits the formation of an extremely thin,yet effective diffusion barrier.

[0049] While not desiring to be bound to any particular theory, itnonetheless is thought that the underlying mechanism by which the metalmonolayer prevents diffusion of the copper, or other highly diffusivemetal, through the barrier layer into the semiconductor or insulatingsubstrate is at least in part attributable to the fact that metal atomsare provided in the monolayer which have relatively large electronclouds which can overlap or touch each other between the metal atoms toeffectively form an energy barrier against movement of copper atomstherethrough. As will be understood by one of ordinary skill in the art,from a standpoint of terminology, the electron clouds are also spoken ofas atomic orbitals occupied by electrons in different energy levels orshells, and the electron cloud is a cloud of negative charge formed ofelectrons of an electron density distribution corresponding to theelement at issue.

[0050] An important advantage of the invention is the ease with whichthe diffusion barrier layer can be formed. Where metal halides are usedas a precursor in forming the diffusion barrier film, the precursor,e.g., BaF₂ or SrF₂, can be deposited for a sufficient duration of timeto ensure complete coverage of the silicon substrate. Such completecoverage can be achieved within relatively short period of time, e.g.,about one minute using MBE deposition of a metal halide on thesubstrate, depending on deposition conditions. Also, the length ofdeposition time is not critical provided it is at least high enough toestablish the diffusion barrier film; deposition times of severalminutes are not detrimental to the procedure, and the depositiontemperature also is not critical. In the second step, all components ofthe precursor except for the monolayer of metal atoms, are removed bythe post-growth annealing procedure. The metal atoms of this thin layeradhere tightly to the substrate, and consequently, the second step canbe carried out over a wide range of time and temperature conditionswithout adversely affecting the formation and character of the diffusionbarrier layer.

[0051] By way of a specific illustration of forming a diffusion barrieron a semiconductor substrate, BaF₂ can be used as the barrier filmprecursor and a silicon wafer can be used as the substrate. The siliconsubstrate first is deoxidized by vacuum annealing at 900° C. for onehour to remove the silicon dioxide passivation layer. Then the substratecan be brought to a deposition temperature of 750° C. in a VG SemiconV8OH MBE growth chamber at a vacuum of less than 1×10⁻¹⁰ mbar. Alltemperature measurements are made from a noncontact thermocouple gauge.A BaF₂ effusion cell can be heated to 1050° C. While the substrateholder is mechanically rotated, an electron beam from a RHEED diagnosticsystem is directed toward the substrate. The beam is focused until theRHEED pattern of a single crystal silicon surface appears on the RHEEDscreen. The shutters in front of the substrate holder and the effusioncell are then opened to allow BaF₂ molecules to impinge on the substratesurface. Deposition of BaF₂ is allowed to continue until the singlecrystal silicon RHEED pattern disappeared and is replaced by a singlecrystal BaF₂ pattern. Deposition is then halted by closing the substrateand effusion source shutters. The substrate temperature is then raisedto 800° C. and held until a RHEED pattern similar to that of the singlecrystal silicon substrate reappears. It will be understood that theabove-provided exemplary protocol is provided merely for sake ofillustration, and not limitation.

[0052] In the mode of the invention being discussed above in which metalhalides are used as precursor compound for forming the diffusion barrierfilm, the precursor compounds that can be used include, for example,BaF₂, BaCl₂, SrF₂, SrCl₂, CsFl, CsCl, RbF, and RbCl, and the like.Especially preferred are those metal halide salts that have cubichalide, e.g. a cubic fluorite, crystal structure. While not desiring tobe limited to any particular theory at this point, applicantsnonetheless consider that precursor compounds obtainable as metalhalides, e.g., BaF₂, BaCl₂, SrF₂, SrCl₂, CsFl, CsCl, RbF, and RbCl, andthe like, that have cubic crystal structure will tend to provide sourcesof metal atoms that are amenable to the above-discussed decompositionreaction and interaction with the silicon surface under readilyimplementable MBE and annealing processing conditions. Although notdesiring to categorically exclude all metal halide salts having rutilecrystal structure, rutile metal halide salts may not be suitable formany processing environments as they do not normally decompose undertypical MBE conditions.

[0053] In another mode of the invention for forming the diffusionbarrier film, the monolayer of metal atoms alternatively can be formedin a one step operation (i.e., without a post-growth anneal step) bydirectly depositing an elemental form of the metal atoms, such asbarium, via MBE on the surface of the semiconductor substrate. Sincecertain elemental metals such as barium are highly reactive, appropriateprecautions have to be taken to handle, maintain and process theelemental barium in an inert environment, e.g., under an argon gasatmosphere, up until it is deposited upon the semiconductor.

[0054] Also, in another embodiment of this invention, it is possible toform the monolayer of metal atoms directly on the semiconductorsubstrate by the above-described two-step decomposition reaction processinvolving a metal halide (i.e., MBE deposit/post-growth anneal), andthen to increase the thickness of the diffusion barrier film bydepositing one or more additional monolayers of metal atoms on theoriginal monolayer through depositing the elemental form of the metalatoms, such as barium, via MBE on the original monolayer. That is, whileformation of a single monolayer on the substrate as described above issufficient to meet the diffusion barrier objectives of this invention,it is also within the scope of this invention to form one or moreadditional monolayers of the metal on the original monolayer as long asthe overarching objective of forming a diffusion layer of extremelysmall thickness is maintained. For example, the metal atom can bedeposited from an elemental form via MBE on the surface of the siliconsubstrate. In this way, a plurality of monolayers can be formed ascontiguous layers upon the substrate to form an overall thickness in thediffusion barrier layer of any desired thickness. Since thin thicknessesare desired, the diffusion barrier preferably is built up to an overallthickness that does not exceed 100 Å, and more preferably does notexceed 20 Å. FIG. 7A illustrates this scenario in which a plurality ofmonolayers 71 a, 71 b, and 71 c are sequentially formed, upon thesurface 72 of substrate 73, one on the other, in the manners describedabove. Then, a conductor material or other material (not shown) can beformed over the outermost monolayer 71 c. In this embodiment, each ofmonolayers 71 a, 71 b, and 71 c are formed of the same type of metalatoms, and together, they form the barrier film. Also, while threemonolayers are depicted in FIG. 7A, the plurality of monolayers can betwo or more.

[0055] Also, it is possible to deposit a combination of different typesof metal atoms during precursor deposition on a substrate to form acomposite diffusion barrier monolayer. For example, because the meltingand sublimation temperatures of strontium fluoride and barium fluorideare similar, the temperature ranges for MBE deposition of a strontiumfluoride precursor onto silicon and for the evaporation of the strontiumfluoride precursor from silicon almost completely overlap those givenabove for barium fluoride. Thus, temperatures in the mid-portion of theranges given for barium fluoride on silicon are also satisfactory forthe MBE deposition and evaporation of strontium fluoride. However, thetemperatures required to sublimate, i.e., directly change the state ofthe source solid crystal form to a gas for deposition via MBE, forbarium fluoride and strontium fluoride are slightly different.Consequently, to control the ratio of barium to strontium in a compositemonolayer of a barrier layer to be formed, the barium fluoride andstrontium fluoride should be deposited using separate effusion cells forthe MBE chamber. In any event, a composite monolayer can be formed ofbarium and strontium atoms in this manner. FIG. 7B illustrates thisembodiment of the invention where the barrier film is a compositemonolayer 71 formed of different types of metal atoms 71 d and 71 e. Twoor more different types of metal atoms can be provided in the compositemonolayer 71. Then, a conductor material or other material (not shown)can be formed over the composite monolayer 71.

[0056] Also, if an additional monolayer or monolayers are deposited onthe original monolayer formed on the surface of the substrate, thedifferent monolayers can have the same or different types of metal atomsby appropriate selection of the precursor compounds at the differentstages of processing. For instance, as illustrated in FIG. 7C, thebarrier film is comprised of a plurality of contiguous monolayers 71 f,71 g and 71 h in which different monolayers thereof are formed ofdifferent types of metal atoms. In this illustration, layers 71 f and 71h are formed of the same type of metal atoms while intervening monolayer71 g is formed of a metal atom that is different from the metal atoms inlayers 71 g and 71 h. However, there is no requirement that barrier filmarrangements with three or more monolayers containing the differenttypes of metal atoms must alternate through the stack of monolayers inany particular pattern. Also, while three monolayers are depicted inFIG. 7C, the embodiment is not limited to that plural number. Also,composite monolayers, such as described in FIG. 7B can be used incombination with one or more contiguous monolayers formed thereon havinga single type of metal atoms, such as shown in FIG. 7A, or differenttypes of metal atoms in different respective monolayers, such asillustrated in FIG. 7C.

[0057] As yet another mode of applying the metal halide precursor to thesubstrate to form the temporary heteroepitaxial film, r.f. sputtering,such as depicted in FIG. 6, can be used. In a sputtering chamber 36, anargon-ion gun 38 directs a beam 40 onto a supply (target) 42 of bariumfluoride, for example, causing deposition of barium fluoride onto asubstrate 44 by sputtering. Here, as in the case of MBE deposition, thepost-growth annealing of the substrate can take place within thesputtering chamber in order to remove BaF₂ molecules and the fluorineatoms leaving only a thin layer of barium atoms as a monolayer adheringto the surface of the substrate. The metallization (conductor) layer canalso be applied to the substrate while it is inside the sputteringchamber. Sputtering can also be used to deposit a diffusion barrier ofother metal atoms, such as strontium atoms, in a similar manner. Also, acombination of different types of metal atoms could be sputtered in thesame monolayer or in different monolayers using different sputteringtargets formed of different respective metal halide precursors. Ingeneral, however, MBE is superior to r.f. sputtering because sputteringcan cause dissociation of the barium-fluorine bond before the bariumfluoride molecule reaches the substrate surface which facilitates theformation of the temporary heteroepitaxial film.

[0058] As other different modes for forming the diffusion barrier filmon a substrate, deposition processes other than MBE and r.f. sputteringcan be used, for example, physical and chemical vapor deposition, wetchemical processes, and liquid phase epitaxy. For instance, precursorsused in metal-organic chemical vapor deposition (MOCVD) to form thediffusion barrier on a semiconductor or insulating substrate include Ba(2,2,6,6-tetramethyl-3,5 heptanedionate) and Sr (2,4-pentanedionate).

[0059] As illustrated in FIG. 8, the diffusion barrier produced inaccordance with the invention can be used not only to prevent diffusionof conductor metals into a semiconductor substrate, but also to preventdiffusion of the conductor metal into an insulating layer. In FIG. 8,layer 46 is a semiconductor substrate, for example, a semiconductinglayer of silicon, and layer 48, which overlies layer 46, is aninsulating layer of silicon dioxide (SiO₂). A plug 45 of a metal, suchas copper, is located in a via hole though insulating layer 48, andmakes ohmic contact with the semiconducting layer 46 through a thindiffusion barrier layer 47 of barium formed by one of the processesdescribed above. This plug is used to conduct current between layer 46and another layer (not shown) which is separated from layer 46 byinsulating layer 48. In the same process, the sidewall of the via holeis lined with a barium diffusion barrier 49, which prevents diffusion ofthe copper into the insulating layer. The barium or strontium atoms aredeposited onto an insulating layer in the same way in which they aredeposited onto silicon.

[0060] As will be apparent from FIG. 8, the minimization of thethickness of the side wall diffusion barrier 49 makes it possible to usecopper for interconnections between layers. The copper interconnects canbe significantly narrower than tungsten interconnects having the samecurrent capacity, and the diffusion barrier is also very thin. Thereforethe use of the diffusion barrier in accordance with the invention as aliner for via holes in insulating layers, can contribute significantlyto the minimization of the lateral dimensions of an integrated circuitof which the elements shown in FIG. 8 are a part. Because the diffusionbarrier 49 is very thin, it permits the use of via holes of relativelylow aspect ratio, making them easier to fill with conducting metal andeliminating voids which result in failures or rejection of ICs.

[0061] It will be understood that this invention is not limited to theabove-illustrated substrate materials, conductor materials, andmaterials used to make the diffusion barrier, as long as other criterionunderstood and set forth herein for these respective materials aresatisfied.

[0062] For instance, the material used for forming the diffusion barriercan be any appropriate metal in elemental form or precursor molecularcompound from which a layer of metal atoms (i.e., a monolayer) can beformed on a semiconductor or insulating substrate.

[0063] The substrate material upon which the diffusion barrier is formedis not particularly limited and can include semiconductor materials andinsulating materials used in semiconductor device fabrications. Thesemiconductor material can be, for example, Si, Ge, InP, GaAs, SiC, GaN,AlN, InSb, PbTe, CdTe, HgTe, Hg_(z)Cd_(1−z)Te, PbSe, PbS, and tertiarycombinations of these materials. The semiconductor material can bemonocrystalline or polycrystalline. The semiconductor substrate can bein bulk wafer form, deposited or grown layer form (e.g., epitaxiallygrown), or silicon-on-insulator (SOI) form. The semiconductor can bedoped or undoped with impurities (e.g., p-, n-doping). The insulatingsubstrate material can be, for example, SiO_(x), SiO₂, BaF₂, SrF₂, CaF₂,silicon nitride, PSG, or BPSG. For example, a thin diffusion barrierfilm formed of a barium, strontium or cesium monolayer (or monolayers)can be used to line via holes in insulators made of BaF₂, SrF₂ and CaF₂.

[0064] As to the types of conductor materials that can be formed on thediffusion barrier, these include conventional metals and metal alloysused for wiring line, interconnects, bonding pads, and so forth, insemiconductor device or opto-electronic device fabrication. The presentinvention is especially useful for providing an in situ barrier toelectrically conductive metals which tend to diffuse into semiconductorand insulating materials common to semiconductor processing. Theseconductive metals include, for example, pure copper, copper alloys(e.g., Cu—Al, Cu—Si—Al), copper doped with a dopant (e.g., aluminum)that impedes electromigration, gold, silver, or platinum. In the case ofcopper, it may be desirable to alloy it with small percentages (e.g.,<5%) of other metallic substances to prevent electromigration. Theconductor material can be deposited on the diffusion barrier by anyconventional technique, including, e.g., electroplating, electrolessdeposition, sputtering, chemical vapor deposition, e-beam evaporation,and so forth. For example, copper can be deposited by e-beam evaporationat 1×10 ⁻⁹ millibars in a heated chamber, or at 10×10⁻¹¹ millibars undera nitrogen environment. The conductor film can be patterned on thediffusion barrier by various techniques, such as by conventionaladditive or subtractive processes known and used in semiconductorprocessing (e.g., photolithographic processing). The invention can alsobe used to prevent diffusion of gallium and/or arsenic from galliumarsenide into silicon and other substrates.

[0065] A number of advantages and improvements are achieved by thepresent invention which can be exploited in the semiconductor deviceprocessing industry. A principal advantage of this invention is that thethickness of the diffusion barrier layer can be made extremely thin. Inpractice, depending on the surface characteristics of the substratematerial onto which the diffusion barrier layer is deposited, thethickness of the diffusion barrier layer according to this inventionwill generally be formed in the range of approximately 5 Å to 100 Å. Inthe case of a smooth, highly planarized substrate material, e.g., asubstrate having a surface roughness well below 5 Å, the diffusionbarrier can be made as thin as one monolayer, which will have athickness slightly less than 5 Å corresponding the atomic diameter ofthe metal atoms forming the diffusion barrier. For such smoothsubstrates, the diffusion barrier formed of one monolayer having athickness less than 5 Å in thickness will satisfactorily inhibitdiffusion of copper and other conductors into the substrate. Withsubstrate materials having a relatively greater surface roughness, thethickness of the diffusion barrier layer will tend to vary. Forinstance, the diffusion barrier layer on a substrate having a surfaceroughness greater than 5 Å may be formed so as to have a thickness valuein the range of approximately 5 to 100 Å, with metal atoms of thediffusion barrier layer accumulating at any step edges on the substratesurface. Thus, the diffusion barrier film of this invention is only onemonolayer or a multiple number of contiguous monolayers formed on thesubstrate surface, and in any event, it need not be more thanapproximately 100 Å in total thickness to achieve the objectives of thisinvention for current and future anticipated semiconductor devicefabrication specifications. A large scale integrated circuit havingcopper conductors and a diffusion barrier film according to thisinvention with a thickness in the range of approximately 5 Å toapproximately 100 Å, can achieve an extremely high component density,which reduces the number of layers required for a given number ofcomponents, and very low heat dissipation. In the practice of thisinvention, therefore, the diffusion barrier thickness can vary from athickness less than about 5 Å to a greater thickness, which can be up toabout 100 Å, preferably up to no more than 20 Å. Conventionalalternative diffusion barriers are significantly thicker than 100 Å.

[0066] Another advantage of the invention is that the diffusion barrierfilm, where it is barium or strontium, or a similar metal, is compliant,i.e., it is mechanically soft and easily deformable. The compliance ofthe diffusion barrier film allows dissimilar materials to be puttogether without introducing defects, such as voids or mechanicalstresses, at the interface which may have detrimental effects on deviceperformance, the diffusion barrier film, or the metallization layer.Furthermore, barium and strontium, or like metals, can formintermetallic compounds with copper (BaCu₁₂ and Cu₅Sr are examples),causing copper atoms to be tightly bound to the barium or strontium atthe interface and unable to migrate past the barium or strontium layerinto the silicon.

[0067] Also, while the barrier film based on one or more monolayers ofmetal atoms that is used in this invention has been illustrated hereinspecifically as a barrier to diffusion of metal conductors intosubstrate materials, it will be understood that the barrier film is notnecessarily limited to that use alone, as it possesses many advantageousattributes that could be exploited in semiconductor device fabrications.For example, the barrier film could be used as a barrier layer in thefabrication of semiconductor laser devices, such as those havingheterojunctions and incorporating different semiconductor materials,e.g., GaAs on top of silicon.

[0068] While the invention has been shown and described with referenceto certain preferred embodiments, it will be understood by those skilledin the art that changes in form and detail may be made without departingfrom the spirit and scope of the appended claims.

What is claimed is:
 1. A semiconductor device comprising: a substrate; abarrier film having a monolayer of barium atoms on said substrate; and amaterial on said barrier film.
 2. A semiconductor device comprising: asubstrate material having a surface; a barrier film on said substratesurface, said barrier film having a monolayer of barium atoms attachedto said surface; a conductor on said barrier film, said conductor havinga tendency to diffuse into said substrate material if in direct contacttherewith; and wherein said monolayer serves as a barrier, inhibitingdiffusion of the conductor into the substrate material.
 3. Asemiconductor device according to claim 2 , wherein said barrier filmhas a thickness of not more than approximately 100 Å.
 4. A semiconductordevice according to claim 2 , wherein said barrier film has a thicknessof not more than approximately 20 Å.
 5. A semiconductor device accordingto claim 2 , wherein said barrier film has a thickness of not more thanapproximately 5 Å.
 6. A semiconductor device according to claim 2 ,wherein said barrier film is a single monolayer of barium atoms attachedto said surface of said substrate material.
 7. A semiconductor deviceaccording to claim 2 , wherein said barrier film comprise a plurality ofcontiguous monolayers of barium atoms located on said surface of saidsubstrate material.
 8. A semiconductor device according to claim 2 , inwhich said substrate material is a semiconductor.
 9. A semiconductordevice according to claim 2 , in which said substrate material is asilicon semiconductor.
 10. A semiconductor device according to claim 2 ,in which said substrate material is an insulating material.
 11. Asemiconductor device according to claim 2 , in which said substratematerial is silicon oxide.
 12. A semiconductor device according to claim2 , in which the conductor is a metal.
 13. A semiconductor deviceaccording to claim 2 , in which the conductor comprises copper.
 14. Aprocess for making a semiconductor device comprising the steps of:forming, on a surface of a substrate material, a barrier film having amonolayer of barium atoms immediately adjacent said surface of thesubstrate material; and depositing a material on said barrier film. 15.A process for making a semiconductor device comprising the steps of:forming, on a surface of a substrate material, a barrier film having amonolayer of barium atoms immediately adjacent said surface of thesubstrate material; and depositing a conductor, having a tendency todiffuse into the substrate material, onto said barrier film, whereinsaid monolayer inhibits diffusion of the conductor into the substratematerial.
 16. A process according to claim 15 , in which the step offorming said barrier film comprises depositing a monolayer precursorcompound on said substrate by molecular beam epitaxy, and then annealingsaid monolayer precursor compound to form said monolayer.
 17. A processaccording to claim 15 , in which the step of forming said barrier filmcomprises depositing a monolayer precursor compound on said substrate bysputtering, and then annealing said monolayer precursor compound to formsaid monolayer.
 18. A process according to claim 15 , in which the stepof forming said barrier film comprises depositing a monolayer precursorcompound on said substrate by physical vapor deposition, and thenannealing said monolayer precursor compound to form said monolayer. 19.A process according to claim 15 , in which the substrate material isselected from the group consisting of a semiconductor material and aninsulating material.
 20. A process according to claim 15 , in which theconductor comprises copper.